Light enters the camera following passage through the atmosphere and reflection off of the
telescope primary and secondary mirrors. The first camera element,
proceeding from right to left, is the dewar's ZnSe window, followed by
a field stop and the first of the seven lenses. This first lens is common to
all three bands. Two dichroic mirrors then split the optical paths for the J, H, and Ks bands.
J-band experiences only a reflection off of the "J-band dichroic." H-band must traverse
the J-band dichroic and then experience a reflection off of the "H-band dichroic." Ks band
traverses both dichroics. Each band then encounters 6 further lenses and a band-limiting
filter. The lens sets are identical in each band.
In Figure 1, the straight-through light path leads to the Ks array, the upper path to the J
array, and the lower one to H.

Figure 1

Telescope Mirror Reflectivity

Figure 2 shows the telescope
mirror reflectivity for a single surface reflection. Starlight
encounters two reflections before entering the camera.

Figure 4 shows the
transmission of a single anti-reflection coated camera lens.
The total transmission for all lenses combined is over
80% (ie, 0.977 approximately),
across each of the three camera bands.

Figures 16 and
17 show atmospheric transmission data for
the north (Mt. Hopkins) and south (CTIO) sites, based
on the USAF PLEXUS code (provided by M. Cohen). In addition to
atomic and molecular absorption, this code accounts for Rayleigh
and site-specific mean aerosol scattering.

The 2MASS J-band
is sensitive to the amount of precipitable water in the atmosphere.
The following plots illustrate the magnitude of the effect for 0.5 mm and
5.0 mm of water vapor, as computed by the ATRAN code.
Figure 18 compares the total system
transmission obtained by the two ATRAN models (Figures
19 and
20) and
overlays the result using the PLEXUS code for comparison.